This invention relates to planar heat generators having improved water tightness and reduced thickness.
The planar heat generator according to the invention is used for, for instance, a water bed for the purpose of heating the bed over a wide area. Planar heat generators of this sort are disclosed in U.S. Pat. No. 4,139,763. Water beds are disclosed in U.S. Pat. No. 4,057,862 and No. 4,107,799. The prior art planar heat generator is usually constructed by sandwiching a meandering heat-generating wire between two insulating sheets made for example of rayon or polyester as intermediate layers, sandwiching the resultant system between two outer insulating sheets made for example of polyvinyl chloride and bonding the individual laminated component parts to one another with a thermal press.
The prior art planar heat generator of this structure, however, has insufficient water tightness, and when it is exposed to water or high humidity for some time, water begins to leak into the interior thereof to gradually increase the dielectric constant, and therefore the insulation property of the heat generator is gradually deteriorated to give rise to an increase in current leakage.
In order to overcome this drawback, it has been tried to use a linear heat generator which has an insulating cover made of a resin having a comparatively high thermal deformation temperature such as Teflon and polyester. In this case, however, the insulating cover of the linear insulator is made of an insulating material having a higher thermal deformation temperature than the material of outer insulator. Therefore, the linear heat generator having the insulating cover is not reduced in diameter by the process of bonding with a thermal press. For this reason, if it is desired to maintain a prerequisite insulation thickness in order to maintain the mechanical and electrical characteristics of the outer insulating sheet, the thickness has to be increased by an amount corresponding to the thickness of the insulating cover of the linear heat generator. This means that the quantity of the material of the outer insulator which has the greatest proportion of the material constituting the planar heat generator is increased to increase cost and also the size of the planar heat generator as a whole.
Further, with the prior art structure, a material having a higher thermal deformation temperature than the outer insulator, for instance Rayon or polyester, is used as the intermediate layer. Therefore, the intermediate layer fails to completely fit to the outer insulating sheet unless the heating condition is sufficient, and it is likely that air is trapped between the two or air bubbles are generated during use, thus causing the peel-off of the outer insulator or reduction of the insulation when the planar heat generator is immersed in water.